The present invention relates to a steam generator comprising an enclosure with a vertical axis containing a bundle of U tubes the ends of which are connected, respectively, via a horizontal tube plate extending transversely in the enclosure, to an inlet collector and an outlet collector for a primary fluid circulating in these tubes, which are bathed on the outside by a secondary fluid taken into the enclosure in liquid form and extracted therefrom in vapour form, a heater in which the secondary fluid is brought to its boiling temperature, located near the tube plate in the region of the tubes, this tube plate being connected to the outlet collector, and an inner casing surrounding the bundle of tubes and surmounted, at its upper end above this bundle, by separating devices for separating the liquid phase carried by the vapour phase, whilst the separated liquid phase is passed into an annular chamber provided between the casing and the enclosure so that this liquid phase comes into contact with the tube plate, and the overall recirculation in the steam generator may be assisted by ejector pumps activated by the secondary fluid as it enters the enclosure.
Various designs are already known for a steam generator of this kind, using a preliminary heater located in the region of the "cold portion" of the bundle of tubes; this heater makes it possible to increase the pressure of the vapour leaving the generator. Steam generators have also been designed which use ejector pumps to produce forced circulation of the secondary fluid in the liquid phase, particularly in the heater, permitting a higher flow rate and consequently an increase in the temperature of the secondary fluid in liquid form, at the tube plate. In particular, generators of this kind have been envisaged more especially in nuclear installations comprising a reactor using pressurised water which constitutes the primary fluid, whilst the secondary fluid also consists of water which is converted into saturated vapour as it passes through the generator.